The role of oxidation states in F-A1 Tln+ (n=1, 3) lasers and CO interactions at the (100) surface of NaCl: An ab initio study

The oxidation states of Thallium in F-A1 Tl+n (n = 1, 3) color centers at the (100) surface of NaCl play important role in laser light generation and adsorbate-substrate interactions. Double-well potentials at these surfaces are investigated by using quantum mechanical ab initio methods. Quantum clusters of variable sizes were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces, and ions that were the nearest neighbors to the F-A1 Tl+n (n = 1, 3) defect site were allowed to relax to equilibrium. The calculated Stokes shifts suggest that laser light generation is sensitive to the oxidation states of Thallium. The relaxed excited states of the defect-containing surface were deep below the lower edge of the conduction bands of the ground state defect-free surface, suggesting that the F-A1 Tl+n (n = 1,3) centers are suitable laser defects. The dependence of the orientational destruction and recording sensitivity on the oxidation state of Thallium is clarified. The Glasner-Tompkins empirical rule is generalized to include the oxidation state of the impurity cation. The adsorption energies of CO and OC over NaCl(I 00) was found to be sensitive to the oxidation state of the impurity cation. F-A1 Tl+n (n = 1, 3) centers changed the physical adsorption of CO to chemical adsorption. While the artificial flow of charge was significant in the case of Tl+1 impurity, it was negligible in the case of Tl+3 impurity, and the results were explained in terms of the electrostatic potential curves. (c) 2006 Elsevier B.V. All rights reserved.